special coloring
matters; such are the bacterium of blue milk, and _Micrococcus
prodigiosus_ (Fig. 2, I.), a red alga that lives upon bread and forms
those bloody spots that were formerly considered by the superstitious as
the precursors of great calamities.

[Illustration: Fig. 2.--VARIOUS MICROBES. (Highly magnified.)]

Another group of bacteria has assumed considerable importance in
pathology, and that is the one whose species inhabit the tissues of
living animals, and cause more or less serious alterations therein, and
often death. Most contagious diseases and epidemics are due to algae of
this latter group. To cite only those whose origin is well known, we may
mention the bacterium that causes charbon, the micrococcus of chicken
cholera, and that of hog measles.

It will be seen from this sketch how important the study of these
organisms is to man, since be must defend his body against their
invasions or utilize them for bringing about useful chemical
modifications in organic matters.

_Our Servants._--We scarcely know what services microbes may render us,
yet the study of them, which has but recently been begun, has already
shown, through the remarkable labors of Messrs. Pasteur, Schloesing and
Muntz, Van Tieghem, Cohn, Koch, etc., the importance of these organisms
in nature. All of us have seen wine when exposed to air gradually sour,
and become converted into vinegar, and we know that in this case the
surface of the liquid is covered with white pellicles called "mother of
vinegar." These pellicles are made up of myriads of globules of
_Mycoderma aceti_. This mycoderm is the principal agent in the
acidification of wine, and it is it that takes oxygen from the air and
fixes it in the alcohol to convert it into vinegar. If the pellicle that
forms becomes immersed in the liquid, the wine will cease to sour.

The vinegar manufacturers of Orleans did not suspect the role of the
mother of vinegar in the production of this article when they were
employing empirical processes that had been established by practice. The
vats were often infested by small worms ("vinegar eals") which disputed
with the mycoderma for the oxygen, killed it through submersion, and
caused the loss of batches that had been under troublesome preparation
for months. Since Mr. Pasteur's researches, the _Mycoderma aceti_ has
been sown directly in the slightly acidified wine, and an excellent
quality of vinegar has thus been obtained, with no fear